The gallery below contains solder defects, primarily beneath array devices such as BGA and QFN packages. Each image features one or more open circuits, due to either process deficiencies (most likely) or material issues (less likely.) Images include both microsections and X-ray, with detailed descriptions and/or discussions. Please visit again, as we are always updating this content with new findings!
All cross section images were captured on Zeiss Axioskop or Axiovert metallurgical microscopes, and X-ray images were captured on an X-Tek Revolution X-ray station, at Process Sciences headquarters in Leander, Texas.
Can you spot the defects?
Look closely at the center of this BGA. Notice that all but one are shaped like little "Rolo" candies? The one in the middle with the rounded bottom did not solder to the board.
Some of these BGAs are connected, some show partial wetting, and the one in the middle is just open.
The inner row of this DQFN package has an open pin. In this picture, it's the bottom row, 2nd from the left.
The stand-off height on this DQFN edge was too much to overcome for one solder joint. Look at the top center, 2nd pin down. A gap is plainly visible between the lead and the bulk solder on the pad.
This open is subtle, because there is no obvious gap between the lead and the pad. Start on the top row, and find the third pin from the right. No wetting angles are evident, but instead the edge of the bulk solder is distinctly visible, overlapping the solder paste beneath like two shadows overlapping under a stadium light.
This is a classic example of a head-in-pillow style defect.
Look at the balls to the right, and above, the marked sphere. Notice the light colored ellipse at the bottom of each? A proper formed solder ball will have two ellipses, one at each pad interface. Second, we know the marked sphere is open because the solder is not reaching the pad. Imagine a tangent line drawn along the bottom row of spheres; this tangent line touches the edge of each pad. All the good joints have wetted right to the pad edge, while the corner ball would not touch that line....because it is not bonded to the pad.
"Deflection" is an industry term for warping of either board or part during the extreme temperatures of the rework cycle. In the image above, severe deflection has prevented most of these spheres from making contact with the board to form proper solder joints. The "good" joints are tall and look more like smokestacks than spheres.
Disaster has struck. Look at the three tall columns of solder on the left, heroically holding up the BGA all by themselves. Those tall solder "spheres" show the actual distance between component and board. There is either severe warping going on, or something has pushed down on the opposite corner causing this corner to rock upward. Or perhaps there's a foreign object preventing the BGA from lying flat. In any case, the majority of these spheres had no chance to form a proper bond.
ALMOST! One misbehaving sphere did not make the connection. Notice the hourglass shape of the corner spheres. There are two reasons for this: 1) the corner pads are much wider (by design) than the interior pads, and 2) some minor warping of the board has created a taller standoff height on this corner. For one ball, it was too much to overcome.
Deflection has resulted in many opens under this BGA. There as many as 17 in this X-ray image.
Insufficient Reflow: The granules visible in these solder paste mounds indicate that the paste has not reached reflow.
The two pins on the right are open. There appears to be enough solder on the pad to form the joint. Perhaps the component was not flat during placement, or required more force to press the pads into the solder paste.
This is another head-in-pillow style open. Flux chemistry has been a challenge since the advent of Pb-free solders, which reflow almost 40 deg C hotter than Sn-Pb. Flux now has to stay activated for longer times and at higher temps...or you get opens like this one.
These BGA spheres are head-in-pillow opens. They appear to be in solid contact with the solder paste, and well aligned, but did not solder. The likely contributing factors are insufficient flux activity in the solder paste, and possible deflection (warping) of board or part. If there was deflection occuring, it seems to have relaxed during cool down.
This defect is very subtle! All of these spheres are aligned just barely off the pad centers, which means the bottom of each sphere is shifted to one side while bonding to the board pad.....except for one. Can you spot the lone exception?
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